Heretofore, a dynamic damper (characteristic variable dynamic vibration absorber) has been widely used to reduce the vibration of a machine. This is a part composed of a spring and a mass having a characteristic frequency which is the same as the frequency of a vibration in question, and reduces the vibration in question by use of: vibrating with an opposite phase to the vibration in question; and the inertia force of the mass.
Here, the following relationship is satisfied:f=ω/2π=(1/2π)√(K/M),
where f denotes a characteristic frequency (natural frequency), ω denotes a characteristic value, K denotes a spring constant, and M denotes a mass.
A dynamic damper typically used vibrates with an opposite phase to that of an input vibration at a characteristic frequency f which is determined by the ratio between a movable mass N and a spring constant K, and uses the inertia force of the mass M in order to reduce the vibration. Hence, one dynamic damper needs to be used for one frequency in question. Further, in the case of using multiple dynamic dampers, if the characteristic frequencies f of these dynamic dampers are close to one another, they might interfere with each other and increase the vibration instead of reducing it.
Meanwhile, in the case of reducing a vibration noise which is a problem caused by the vibration of an engine, several frequencies in question sometimes exist due to variation of a frequency in question in synchronization with an engine speed.
Thus, in order to achieve a dynamic damper whose characteristic frequency f is variable, a dynamic damper using a magneto-rheological elastomer (MRE) as a spring is proposed (see International Patent Application Publication No. WO2012/026332, for example). This is a technique for varying the rigidity of a rubber, mixed with magnetic substances, by controlling the strength of a magnetic field generated by an electric current flowing through a coil.